Ambiguity errors are generally resolved by
1. rewriting the content model in a non ambiguous way
2. relaxing the content model and place the constraints at some other
level

In your case your complex type can be rewriten in a non ambiguous way
as:

  <xsd:complexType name="kundensucheTyp">
    <xsd:choice>
      <xsd:sequence>
        <!-- Suche mit Name / Anschrift -->
        <xsd:element name="vorname" type="xsd:string" minOccurs="0"/>
        <xsd:element name="nachname" type="xsd:string"/>
        <xsd:element name="strasse" type="xsd:string" minOccurs="0"/>
        <xsd:element name="ort" type="xsd:string" minOccurs="0"/>
        <xsd:element name="geburtstag" type="datumTyp" minOccurs="0"/>
      </xsd:sequence>
      <xsd:sequence>
        <!-- Suche mit Vertragsnummer  -->
        <xsd:element name="vertragsnummer" type="vertragsnummernTyp"/>
      </xsd:sequence>
    </xsd:choice>
  </xsd:complexType>

Best Regards,
George
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George Cristian Bina
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
www.---.com